Circuit boards or cards are used in numerous electrical devices for mounting electrical components near each other, and electrically inter-connecting those components. When more components are required than can fit on one board, additional boards will typically be used, with one or more electrical connections among boards. An edge connector arrangement allows boards to be placed parallel to each other, or "stacked". This placement allows for numerous boards to be placed in a relatively small space.
There have been numerous edge connector arrangements employed in the art. Descriptions of several such devices may be found in U.S. Pat. Nos. 3,609,463; 3,922,054; 4,288,139; and 4,603,928. Several of such devices utilize a conventional "push-in" edge connector assembly whereby a connector is assembled to a mother board with wires extending between the contact terminals in the plastic housings of the connectors and contacts on the mother board. In this type device, the terminals of the connector frictionally wipe against contacts on a daughter board as it is being inserted into the connector. Such wiping may cause harmful wear. Due to this problem, more recent devices utilize low insertion forces during the mating process, with means for increasing the force applied to the board by the connector after the mating is achieved. Most recently, Zero Insertion Force (ZIF) methods have been utilized, whereby no force is exerted on the board contacts while the board is being inserted or removed. In operation, the board is inserted into the connector while the terminals are in a retracted position, and then a movable cam is actuated to press the connector terminals against the board contacts. The terminals are disengaged from the board by reversing the camming process to allow removal of the board.
Rubber and elastomeric materials, with alternating conductive and non-conductive segments or layers, have previously been used in board connection devices. Such a device is described in U.S. Pat. No. 4,639,062. That device utilizes a rigid rubber structure to hold a board edge while simultaneously forming an electrical connection. Malleable elastomers have been utilized as layers in board assemblies because of their thinness and shock-absorbent qualities, but not as board holding members, due to their lack of rigidity.
Conductively segmented materials are commercially available as standard items in the market. The conductive segments are typically formed by incorporating carbon or metal particles in the elastomeric material, wherein the conductive layers are insulated from each other by the alternate layers of non-conductive elastomeric material.
The difficulties inherent in the prior art interconnection devices are: the necessity for conductive paths through the holding members, which decreases the speed of the device; the physical size of the overlapping portion of connector elements, which lessens the available space on the surfaces of the boards for the placement of electrical components; and, in the non-elastomeric or rubber embodiments, the limitation on the number of connections that will fit on each board, due to the bulkiness of the connectors.
The most serious of these difficulties is the length of the conductive path between boards. Lengthening the path exacerbates two problems, both related to the attainable electrical speed of the overall device.
The first problem is delay, which is reduced in accordance with the reduction in conductive distance. The second is impedance imbalance. To the extent that the impedance of the connector does not match the impedance of the circuit on the circuit board, the detrimental effects of the imbalance are reduced as the length of the connector conductive path is reduced. The end result of the amelioration of these problems is the ability to achieve higher speed electrical operations through the connector.